Had the same concerns as you and I started work right around when he turned five months. I believe it's just your supply adjusting to his needs. In my case I was producing 20-24 ounces in three pumping sessions and then it dipped to 16. Now it has evened out at 17-18 and he hardly ever eats that much when I am away. Don't stress as it can cause a dip and just drink plenty of fluids and have oatmeal once in a while with flax and brewers yeast. It helped me I think. I also did lactation cookies and mother's milk tea a lot at that time. And make sure to nurse a lot when at home on weekends. Although I wonder if it would have been the same result had I just nursed more and drank fluids. I still nurse (or offer) in the morning before I go to work and when I come home when he wakes from his second nap. Then for bedtime and once or twice in the night.

If your period really is on the horizon, then your hormones could be messing with your supply, in which case any dip is temporary.

I would try to nurse him before bed, then give him the bottle of formula, then pump. Or if you want extra pumped instead, try an extra pumping session before you go to bed. Otherwise I'd drink an extra couple of glasses of water (or more, if you can). I can tell my supply suffers when I don't drink enough water.

Are you eating enough? I'd guess you are but just want to throw that out there

Also, you can drink some mother's milk tea, eat lactation cookies, and/or eat some extra oatmeal. However the biggest thing for me to help my supply is more water.

Keep us posted!

I'm not sure what your weight loss journey is like, but are you eating enough? You need at least 500 calories a day extra when nursing/pumping full time. When I was trying to lose weight when my son was 5 months old, my supply did drop quite a bit and it stressed me out so I quit. I think I probably wasn't eating enough, even though it was close to 2000 calories a day! And of course drinking enough water is important too.

If you feel like you are eating enough, it can also be very typical for your supply to drop after you've been pumping for a couple months. Your supply has probably regulated and with your baby sleeping through the night, it's dropped off even more. I pumped for 3 babies at work up until each of them turned 1 and this happened with all of them. What I did with my second two kids (I didn't do it with my first and wish I had) was started to pump in the morning before I left for work. Sometimes I would pump from one side while nursing baby on the other (which helps to stimulate a letdown). Most of the time I was able to pump a full bottle of milk in the morning to use for that day, plus it helped keep my supply up more. I did that every day of the week, even on the weekends. You could also do an extra pumping session at night, but I found that my supply was much higher in the morning than at night so I got more milk from pumping in the morning.

My supply dipped a little when I started dieting... I did fenugreek for two weeks until it was back up. Takes a few days to start working, and then some women say that their supply goes down again after not taking fenugreek, but never happened for me.

Been dieting for a month now, and haven't had any other issues. But I do add extra calories for breastfeeding too, and eat back all exercise calories.